Atmospheric carbon dioxide concentrations in the southern hemisphere have breached the symbolic 400 parts per million mark.

From the World Meteorological Organization (WMO)

Carbon dioxide concentrations in the southern hemisphere have joined those in the northern hemisphere and breached the symbolic 400 parts per million (ppm) mark – and are unlikely to dip back below this level for many decades to come.

Measurements of carbon dioxide (CO2) at Cape Grim on Tasmania’s northwest coast reached the milestone on 10 May and at Casey Station in Antarctica on 14 May, according to CSIRO and the Bureau of Meteorology of Australia. The stations form part of WMO’s Global Atmosphere Watch network, which spans 100 countries, including stations high in the Alps, Andes and Himalayas, as well as the Arctic and Antarctic. The threshold was reached earlier than anticipated, as a result of a recent, strong increase in the growth rate of atmospheric CO₂. This was probably driven by increased emissions from fossil fuels, as well the impact of the recent strong El Niño, which reduced the capacity of natural systems such as oceans and plants to absorb (CO2), according to Paul Krummel, Research Group Leader CSIRO, and Paul Fraser, Honorary Fellow, CSIRO, writing in the Conversation.

Carbon dioxide concentrations

CO2 concentrations over the southern hemisphere are trailing those in the planet’s northern half, where 400 ppm level was breached in 2014-15. The northern hemisphere’s carbon dioxide levels are higher because most CO2 sources (fossil-fuel-burning installations) are mainly found in the north, whereas CO2 “sinks” such as oceans are predominantly in the southern hemisphere.

The northern hemisphere’s CO2 levels also show a much stronger seasonal variation due to large land mass (and related biosphere). According to the U.S. National Atmospheric and Oceanic Administration, the global monthly mean CO2 level in March 2016 was 403.94 ppm, up from 400.36 ppm in March 2015.

“The impact of El Niño on CO2 concentrations is a natural and relatively short-lived phenomenon,” said WMO Secretary-General Petteri Taalas. “But the main long-term driver is greenhouse gas emissions from human activities. We have the power and responsibility to cut these.” “This should serve as yet another wake-up call to governments about the need to take urgent action to make the cuts in CO2 emissions necessary to keep global temperature rises to well below 2°C,” he said.

The news coincided with the opening of the UN Climate Change conference in Bonn. This is focussing on how to implement the Paris Climate Change Agreement.